test_common: factor out keySalt computation
test_common: factor out keySalt computation

Suggested by @akalin-keybase

Issue: #759

keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs